Ovens and other heating appliances which use quartz halogen lamps as the source of radiant energy for heating objects are known. Such ovens typically include a plurality of quartz halogen lamps which are arranged in parallel and adjacent to the ceiling and/or floor of the oven. When the lamps are energized, they emit high power density radiant energy. The heating of objects, such as food, within these ovens results predominantly from this high power density radiant energy. The filaments of these lamps are low in mass and may be operated at very high temperatures (e.g., at about 3000 Kelvin). These characteristics allow food to be cooked quickly with infrared radiation, while not requiring any pre-heating of the oven.
However, each quartz halogen lamp includes one or more terminals, that are used to connect the lamp to a source of electrical energy, and that must be kept at a temperature below 350.degree. C. Above this temperature, seals in the terminals leak and ingest air at an excessive rate, leading to premature failure of the quartz halogen lamp. Therefore, the terminals of the quartz halogen lamp must be cooled to ensure proper operation and long life.
The most common cooling method is to pass air directly over each quartz halogen lamp. Each quartz halogen lamp typically includes an elongated quartz sleeve that encloses a tungsten filament. By passing air over the quartz sleeve, the terminals of the quartz halogen lamp are cooled indirectly. The heat transfer mechanism used in this cooling method is commonly known as forced convection heat transfer. Forced convection heat transfer is governed by the following equation (Newton's law of cooling): EQU Q=h.sub.c A(T.sub.h -T.sub.c)
where: Q is the rate of heat transfer (BTU/minute); h.sub.c is a convection heat transfer coefficient that is a function of fluid properties, flow field and surface properties of the object being cooled; A is the effective surface area (i.e. the outer surface area of the cylindrical quartz sleeve) ; T.sub.h is the temperature of the hot surface (i.e. the cylindrical quartz sleeve outer surface); and T.sub.c is the temperature of the colder medium (i.e., the cooling air).
Forced convection heat transfer rates are difficult to quantify, mainly due to the difficulty in determining the magnitude of the convection heat transfer coefficient. However, as the cylindrical quartz sleeve has a relatively small surface area, the rate of heat transfer achieved by passing air directly over each quartz halogen lamp will also be proportionally small. As a result, the temperature of the quartz halogen lamp terminals will be higher than desired, unless the cooling air is at a low temperature and/or is passed across the quartz halogen lamp at a very high mass flow rate.
An additional drawback of forced convection cooling of quartz halogen lamps is that air passing over the lamps introduces airborne dust and grease, that will contaminate the outer surface of the cylindrical quartz sleeve, and that will thereby shorten the useful life of the lamp. (To avoid premature failure, manufacturers of quartz halogen lamps recommend that even small amounts of contamination, such as may be caused by fingerprints, for example, be kept away from the surface of the quartz sleeve of a halogen lamp.)
Accordingly, it is desirable to cool quartz halogen lamps without impinging air directly on the lamp surfaces, especially in an environment such as an oven that has relatively high concentrations of contaminants, such as grease and dust in the air within and around the oven.
The present invention is directed to an apparatus for cooling quartz halogen lamps which solves one or more of the above-noted problems. The invention is particularly advantageous when used in a heating appliance, such as an oven.